The present invention relates to a method for controlling the antennas of a mobile terminal device that has a plurality of antennas, for example. More particularly, the present invention relates to a method for controlling antennas in accordance with the usage of such a mobile terminal device and to such a mobile terminal device.
In recent years, the effects of electromagnetic wave emitted from mobile terminal devices on the human body has been a subject of considerable interest, and technologies for reducing absorption of electromagnetic wave into the human body have become an issue in designing mobile terminal devices. For example, in the United States, a standard has been established that prescribes the acceptable value of Specific Absorption Rate (SAR), which is a measure of electric energy of electromagnetic wave absorbed by the human body, to be under 1.6 W/kg.
In such a circumstance, for a mobile terminal device that has a plurality of antennas for wireless communication, a method has been proposed for always selecting an antenna with better SAR properties for use from the plurality of antenna on such a device. For example, Japanese Unexamined Patent Publication No. 2003-283393 discloses a method for switching between “a first antenna that is designed to obtain preferable SAR properties in all usages” and “a second antenna that is not so designed” and using the selected one in accordance with how a mobile terminal device (e.g., a mobile phone) is being used for the purpose of reducing SAR associated with a mobile terminal device.
Specifically, the method switches between the first antenna and the second antenna according to the usage of the mobile terminal device, such as whether the external data terminal is in use or not, whether handsfree communication is being carried out or not, whether or not the external audio communication terminal is in use, whether the camera is in use or not, whether image communication is being carried out, or whether packet communication is being carried out. In addition, a technique that is based on a method for determining the distance between the mobile terminal device and the human body and a technique based on a method for detecting opening/closing of the housing of the mobile terminal device have been also disclosed. Thus, the method disclosed by Japanese Unexamined Patent Publication No. 2003-283393 is intended to improve SAR properties while avoiding degradation of communication speed and quality of a mobile terminal device as much as possible.
However, another issue arises when such a method is applied to a laptop PC or handheld PC, for example.
More specifically, a mobile phone can have various usages, that is, a mobile phone may be used as a data communication terminal, audio communication terminal, camera, image communication terminal, packet communication terminal and so on as mentioned above. Thus, it is necessary to employ a complex control method such as the one described above to select an antenna according to such usages. In addition, such a device has to be small in size for use as a mobile phone. For those reasons, even if a terminal has a plurality of antennas, when the terminal is used in close contact with the human body such as for voice call, it could be difficult to reduce SAR unless transmission output itself is reduced for any antenna at any position selected. Thus, in such a case, with the method disclosed by Japanese Unexamined Patent Publication No. 2003-283393, it is necessary to select an antenna that is designed to limit transmission output in order to maintain preferable SAR properties. In this case, degradation of communication speed and quality of the mobile phone has to be accepted.
On the other hand, a laptop PC or handheld PC does not have so many usages as a mobile phone. In addition, size of such mobile terminal devices for use as a terminal is less restricted. With these features, control could become too complex if the antenna control method for a mobile phone terminal device described above is applied to a laptop PC or a handheld PC without modification. To realize such complex control, many devices such as sensors are required for detecting how the terminal device is used. As a result, the mechanism of a mobile terminal device itself becomes complex and there would be more causes of trouble accordingly. In addition, when an antenna with its output limited low is selected, communication speed or quality of the mobile terminal device degrades.
In view of such circumstances, it is desirable to provide a method for controlling antennas without degrading communication speed or quality of a mobile terminal device such as a laptop PC and a handheld PC as well as a mobile terminal device having such control means.